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Key takeaways
- Layer 2 networks process transactions off the main chain and post the results back to it, cutting fees while borrowing much of the base layer's security.
- Optimistic rollups assume transactions are valid and allow a challenge window, so official withdrawals take about a week; ZK rollups prove validity with cryptography instead.
- Most layer 2s still rely on operators, upgradeable contracts and centralized sequencers; L2BEAT's Stages framework is a useful way to check how much trust each one requires.
A layer 2 is a network built on top of a main blockchain that handles transactions more cheaply and quickly, then records the results back on the main chain. On Ethereum, most layer 2s are rollups, and how much you must trust each one varies widely.
Why layer 2s exist
A layer 1 blockchain like Ethereum or Bitcoin is deliberately limited in how many transactions it can fit into each block. Keeping blocks small lets ordinary people run nodes and verify the chain, which keeps it decentralized. The trade-off is that when demand is high, fees climb.
A layer 2 moves most of the work off the main chain. Thousands of transactions are executed elsewhere and then summarized on layer 1. As ethereum.org explains, the goal is to scale while relying on the base layer for security, rather than building a whole new set of validators from scratch.
Ethereum made this cheaper with its Dencun upgrade in March 2024, which introduced “blobs,” a dedicated and lower-cost space for rollups to post their data.
Rollups: optimistic vs ZK
A rollup bundles many transactions, posts the data to Ethereum, and submits a claim about the new state of the network. The two main families differ in how that claim is checked.
Optimistic rollups
An optimistic rollup assumes each batch is valid unless someone proves otherwise. Anyone watching can submit a fraud proof during a challenge window. If the challenge succeeds, the bad batch is thrown out. Arbitrum One, OP Mainnet and Base work this way.
ZK rollups
A ZK rollup attaches a validity proof, a piece of cryptography that shows the batch was computed correctly without re-running it. Ethereum verifies the proof directly, so no challenge window is needed. According to ethereum.org, this can allow faster withdrawals, at the cost of complex and computationally heavy proving systems. ZKsync Era and Starknet are well-known examples.
Sequencers
On most rollups, a single sequencer collects transactions, puts them in order and produces blocks. This is why layer 2 transactions often feel nearly instant. It is also a point of centralization: the operator usually runs the sequencer, and it could in principle delay or censor transactions or go offline.
Well-designed rollups include an escape hatch, letting users submit transactions or withdrawals directly through Ethereum if the sequencer misbehaves. Whether that works in practice is one of the things independent reviewers check.
Bridges and withdrawal times
To use a layer 2, you move funds onto it through a bridge. Many exchanges now let you withdraw directly to popular layer 2s, which skips the bridging step. If you do send to a layer 2, pick that exact network as described in our guide to sending crypto safely.
- Deposits from Ethereum to a rollup usually arrive within minutes.
- Withdrawals from optimistic rollups through the official bridge must wait out the challenge window. Arbitrum's documentation sets the default at roughly one week, and OP Stack chains use a similar period.
- Withdrawals from ZK rollups can finalize once a proof is verified on Ethereum, which is typically faster but varies by network.
- Third-party bridges can get you out faster by fronting you funds, but you pay a fee and trust another system.
L2BEAT stages: how much trust is involved
Not every rollup is equally independent from its operators. L2BEAT, an independent research group, introduced its Stages framework in June 2023 to grade this.
- Stage 0: the project is still largely controlled by a few parties, although its data and code must be available.
- Stage 1: “limited training wheels.” A working proof system exists, and users can exit even if operators misbehave, but a security council can still override the system in emergencies.
- Stage 2: the rollup is governed by code. Intervention is restricted to provable bugs, and users get long notice before upgrades.
As of October 2026, L2BEAT's summary lists Arbitrum One, Base, OP Mainnet and Starknet at Stage 1 and ZKsync Era at Stage 0. These ratings change, so check the live page. L2BEAT itself stresses that stages measure decentralization, not overall safety; a Stage 2 project with buggy code can still fail.
Examples of layer 2 networks
| Network | Type | Notes |
|---|---|---|
| Arbitrum One | Optimistic rollup | Developed by Offchain Labs; governed by holders of the ARB token |
| Base | Optimistic rollup | Incubated by Coinbase and built on the OP Stack |
| OP Mainnet | Optimistic rollup | The original Optimism chain; its OP Stack code powers many other chains |
| ZKsync Era | ZK rollup | Built by Matter Labs; EVM-compatible |
| Starknet | ZK rollup | Built by StarkWare; uses STARK proofs and its own Cairo programming language |
| Lightning Network | Payment channels on Bitcoin | A different kind of layer 2 for fast, low-cost bitcoin payments |
Polygon is often grouped with layer 2s, but L2BEAT describes its main chain, Polygon PoS, as a proof-of-stake sidechain with its own validators, so it does not inherit Ethereum's security the way a rollup does.
Bitcoin's Lightning Network
The Lightning Network takes a different approach. Two users lock bitcoin into a shared channel with an on-chain transaction, then exchange signed balance updates privately as often as they like. Payments can hop across connected channels. Only opening and closing channels touches the Bitcoin blockchain. It is built for payments rather than general-purpose apps, and it requires users or their wallet providers to keep channels funded and online.
Risks to keep in mind
- Upgradeable contracts: on many rollups, a small group can change the bridge contracts that hold user funds.
- Sequencer outages: a halted sequencer can freeze activity until it recovers or users fall back to slower escape routes.
- Proof system bugs: fraud and validity proofs are complex, and a flaw could allow invalid withdrawals.
- Bridge exploits: third-party bridges add their own smart-contract and operator risk.
- Fragmentation: the same token on different layer 2s is not automatically interchangeable, which makes sending to the wrong network easy.
Where to go next
This lesson builds on buying crypto safely, sending crypto safely, DeFi and staking. Finish this stage of the course with our stablecoins guide.
Frequently asked questions
Is a layer 2 a separate cryptocurrency?
A layer 2 is a separate network, but on Ethereum rollups you typically pay fees in ETH. Some layer 2s also have their own governance tokens, such as ARB for Arbitrum, which are not required to use the network.
Why does withdrawing from some layer 2s take a week?
Optimistic rollups give anyone a window, roughly seven days on Arbitrum and OP Stack chains, to challenge an invalid state before withdrawals through the official bridge finalize. Third-party bridges can be faster because a liquidity provider fronts you the funds, in exchange for a fee and extra trust.
Is Polygon a layer 2?
It depends which Polygon network you mean. L2BEAT describes Polygon PoS, the main Polygon chain, as a proof-of-stake sidechain with plans to become a validium, rather than a rollup that inherits Ethereum's security.
Is the Lightning Network a layer 2?
Yes, but a different kind. Lightning uses payment channels between users rather than rollups, letting people send bitcoin quickly and cheaply off-chain and settle on Bitcoin's main chain when channels close.
Sources
- What is layer 2? — ethereum.org
- Zero-knowledge rollups — ethereum.org
- The Stages Framework — L2BEAT
- Layer 2s summary — L2BEAT
- Customizable challenge period — Offchain Labs (Arbitrum Docs)
- Polygon PoS project page — L2BEAT
Updated October 4, 2026 by The Crypto Guide editorial team. Educational content, not financial, legal or tax advice. Spot an error? Request a correction.